ALLEN PARK >> The interior of the Detroit Lions’ offensive line was set to have a pretty intriguing competition — and then All-Pro center Frank Ragnow announced his retirement in early June, turning a battle into an all-out war.

On a roster with very few question marks, there are three open spots right next to each other on a unit that has been Detroit’s calling card.

Still, there’s a qualified pool of candidates ready to compete for starting jobs. While the situation seems a bit dire from the outset, there is still a reasonable cause for optimism as the Lions start trying to figure out life post-Ragnow.

“I mean, look, it’s different knowing that Frank’s out of the fold now. It is,” Lions coach Dan Campbell said a few days after Ragnow’s announcement. “There is acknowledgement, like, ‘OK, so that’s where we’re at, and now we need growth. We need development, quickly.’”

One thing offensive line coach Hank Fraley has preached over his tenure in Detroit is that competition brings out the best in players.

While news of Ragnow’s retirement had yet to break when Fraley was talking about the upcoming camp battle last month, it can be safely assumed he knew losing Ragnow was a possibility.

“Competition brings out the best in you and the best of the unit,” Fraley said. “It will weed out the weak, too, the guys who just can’t keep up. Talking with all our guys this year, there’s a lot of opportunity for everybody.”

As the Lions try to keep the train chugging under new offensive coordinator John Morton, figuring out the offensive line situation will be essential.

“If we can’t run or protect, all these plays we draw up, it doesn’t mean anything if we don’t protect the quarterback, and running the ball helps protect the quarterback,” Morton said. “It’s what they’ve done here, and that’s what we’re going to do.”

Left tackle Taylor Decker and All-Pro right tackle Penei Sewell have the bookends of the line locked down. But inside, all bets are off.Incumbent veteran starter Graham Glasgow, who played left guard last season but has also played right guard and center for the Lions, is one of the front-runners to win a job, but his place is by no means guaranteed.

Behind him there’s Christian Mahogany, who started two games as a rookie, and newly drafted rookies Tate Ratledge and Miles Frazier will also be vying for a spot.

Other interior players who have been reserves in the past, such as Kayode Awosika, Colby Sorsdal, Michael Niese and Kingsley Eguakun, will have every opportunity to prove themselves worthy of a bigger role.

There are a plethora of potential starting combinations, and Fraley said the team is simply looking for the best group of five linemen, regardless of where each player has the most familiarity.

“We’re trying to play our best five, and how do those chips fall? … I’ve been in their same shoes when you draft somebody first round, second round,” Fraley said. “It’s just, ‘OK, I’m coming in, I’m going to work hard, do what I can do, take care of what I can take care of.’ It’s only going to, normally, make you a better player, and it’s going to make those guys around you better.

“So, I think we just elevated our level of play this year.

“Now we’ve got to go out and do it.”

Fraley is entering his eighth season with the franchise, making him one of the longest-tenured members of the staff alongside assistant linebackers coach and senior defensive assistant David Corrao.

“He’s done a hell of a job developing talent.

“He’s going to get these guys right — whoever it is — between Frazier and Ratledge,” Campbell said of Fraley.

“And there again, having Graham helps now — it’s huge — because he’s got the flexibility to play both guards and center.”

Fraley garnered interest from Seattle for the Seahawks’ vacant offensive coordinator role but ultimately opted to stay in Detroit, a massive win for a regime that endured a significant brain drain.

Now, he’ll have to piece back together the unit that has anchored the Lions to consecutive NFC North division titles.

“I just want to be the best O-line coach I can be, to be honest,” Fraley said, noting that the chance to interview for a bigger role was “a great experience.”

“I’m very glad I’m back here because this is where I wanted to be with my family.

“We’ve got special stuff here, I’ve got a special room.”